The present invention relates to a method for processing the output signals of an optoelectronic scanner in a replay or recording apparatus for replaying from or recording onto an optical recording medium, as well as a corresponding appliance having an input circuit of a digital servo controller, suitable for carrying out the method.
Both such a method and the said appliance having an input circuit of a digital servo controller have been known for some time as components of optical disk drives for CDs, magneto-optical disks, CD-Roms, digital video disks etc. In this case, the servo controller is used to ensure that the laser beam which scans the optical recording medium always follows the track of the pits on the recording medium accurately and, despite the optical recording medium having a certain amount of unevenness which cannot be avoided, eccentricity or other disturbance influences are focused at any time with adequate precision onto the information-bearing layer of the recording medium. In the case of the known methods and the corresponding appliances, the output signals of the scanner are first of all supplied to an analogue circuit with a low-pass filter and a trimming and correction circuit, and the focus error signals resulting from this, as well as the track error signals which characterize a track deviation are then digitized at the normal, fundamental sampling frequency of 44.1 kHz or else at twice the sampling frequency, which is likewise normal, of 88.2 kHz in an analogue/digital converter, called an A/D converter in the following text. After this, the digitized signals are made available to the digital servo controller as input values.
The disadvantages of the known method and of the corresponding known appliance are, in particular, that the analogue low-pass filter is essential for suppressing high frequency components, e.g. radio-frequency components, of the two error signals, and that the necessary trimming or the corrections required of OFFSET and BALANCE must be carried out in a relatively complex manner in the analogue domain. The known method and the corresponding circuit of a known appliance possibly require a number of stages of analogue low-pass filters before the A/D conversion, and trimming or correction can be carried out digitally only if each output signal of the optical scanner is digitized separately. Thus, neither the known method nor the corresponding appliance is optimum for integration of a decoder and of a digital servo section, containing an A/D converter and a digital servo controller, e.g. using CMOS technology.